diff -Naur a/build/moz.configure/warnings.configure b/build/moz.configure/warnings.configure
--- a/build/moz.configure/warnings.configure	2022-05-27 02:16:52.000000000 +0300
+++ b/build/moz.configure/warnings.configure	2022-05-31 10:48:22.990378071 +0300
@@ -293,7 +293,8 @@
 check_and_add_warning("-Wno-gnu-zero-variadic-macro-arguments")
 
 # Make it an error to be missing function declarations for C code.
-check_and_add_warning("-Werror=implicit-function-declaration", c_compiler)
+check_and_add_warning("-Werror=implicit-function-declaration",
+                         when="--enable-warnings-as-errors")
 
 # New in clang 11. We can't really do anything about this warning.
 check_and_add_warning("-Wno-psabi")